There are a variety of wheel slip control systems whose construction and operation are well known in the rail transport industry. The common objective of all wheel slip control systems is to vary the force that the brakes apply to the wheels of the rail vehicle during braking so that the wheels neither slide nor lockup as they travel on the top surface of the railway track. By adjusting the force with which the brakes apply so as to prevent or at least reduce slipping, a brake control system can more safely and efficiently decelerate and stop a rail vehicle in a shorter distance than would be possible without the use of a wheel slip control system.
Wheel slip control systems essentially control the interaction between the wheels and the tracks on which they ride. The top surface of the rails on which the tread of the wheels ride is typically called the running surface. The wheel treads adhere to the running surface of the rails essentially by means of friction. Under any given set of operating conditions, there exists between a wheel tread and the running surface on which it rides a particular level of frictional adhesion. It is this friction between tread and running surface which allows the wheel treads to have traction on the rails as the vehicle travels along the tracks. Should the braking force applied to the wheels exceed that which can be sustained by the maximum amount of frictional adhesion inherent to the particular wheel tread-running surface environment at issue, the treads will no longer completely grip and thus slip, and maybe even slide, on the running surface. It is such slipping and sliding that wheel slip control systems attempt to eliminate or at least reduce.
The wheel slip control systems in the art today typically employ one or more microprocessors accompanied by a requisite number of memory storage devices. These memory devices may, of course, be either separate from or actually embodied in the microprocessor(s) (hereinafter "microprocessor"). The microprocessor executes a specific set of instructions contained in programming code. The programming code, according to whose directions or logic the wheel slip control system operates, is stored in these memory devices. The microprocessor and associated memory devices are typically housed within a single package referred to as the wheel slip controller. It is this controller that controls various other parts of the wheel slip control system as explained hereinafter.
The memory storage devices may also be used to store numerous tables of parameters or a number of individual parameters or both. These parameters are constants, with variable values, each of which is preselected to work in connection with the circumstances of its application. In other words, these parameters, whether individually or as part of a table, are generally "setup" or tuned to each individual application. As the microprocessor executes the instructions contained in the programming code, the programming code generally requires the microprocessor to retrieve one or more of the parameter values from memory to complete a particular task. Exactly which value in a parameter table, for example, the microprocessor retrieves depends on the conditions under which the vehicle is then operating. The parameters along with the programming code are required by the microprocessor to operate the wheel slip control system. The parameter values serve as referents for determining other variables in the process through which to control slipping of the wheels.
Wheel slip control systems ideally prevent or at least reduce slipping by detecting nascent slipping and then correcting accordingly the force applied by the brakes to the wheels of the truck on the rail vehicle. Simply described, after the wheel slip is detected, the wheel slip control system corrects (i.e., temporarily reduces) the braking force applied to the slipping wheel until the rotational speed of the wheel and its associated axle again matches the speed of the rail vehicle. Once the rotational speed of the wheel/axle combination matches or perhaps briefly exceeds the speed of the rail vehicle, the wheel slip control system reapplies the braking force to the wheel. This process of detecting and correcting wheel slip occurs rapidly and generally continuously on most all of the wheel slip control systems in the art.
The system described in U.S. Pat. No. 4,071,282, entitled SLIP-SLIDE DETECTOR SYSTEM FOR RAILWAY CAR WHEELS, uses data from all of the axles on the vehicle to control wheel slip. This system, however, does not compensate for differences in diameter among the wheels. Depending on how much wear each wheel may have experienced, the wheels on the vehicle may exhibit significant differences in diameter. Consequently, the axles may rotate at different speeds. The system may then detect wheel slip even though in fact the problem may be simply due to differences in wheel diameter. More recently developed wheel slip control systems such as those alluded to below have overcome this problem.
A wheel slip control system typically controls such slipping through a wheel slip control valve. The slip control valve may be a distinct component that functions only in conjunction with the wheel slip control system or the same component that is used by the brake control system to modulate the braking during normal vehicle braking. The slip control valve is typically connected to the brake cylinder(s) or brake cylinder supply line(s) of the truck. The slip control valve on most systems is usually a dual solenoid valve capable of being commanded to assume any one of three positions. When commanded by the wheel slip controller to a release position, the slip control valve vents pressure from the brake cylinders so as to release the brakes on the truck. When commanded to lap position, the valve holds constant whatever pressure is currently in the brake cylinders. When commanded to an apply position, the slip control valve supplies pressure to the brake cylinders so as to reapply the brakes on the truck.
The operation of a slip control valve in one type of wheel slip control system is described in U.S. Pat. No. 4,491,920, entitled RATE POLARITY SHIFT WHEEL-SLIP CONTROL SYSTEM. This patent is assigned to the assignee of the present invention, and incorporated by reference into this document. This particular system detects wheel slip by monitoring the rate at which each wheel/axle combination decelerates during braking. A rate determining circuit on each truck determines which wheel/axle combination on the truck exhibits that highest (most negative) rate of deceleration. When the rate of deceleration increases below a first predetermined (negative) threshold, the wheel slip controller commands the slip control valve to the release position thereby reducing the force applied by the brakes to the wheels of that truck. This reduction in braking force allows the deceleration rate of the wheel/axle combination to change from being increasingly negative in value to decreasingly negative in value as the axle again approaches the speed of the vehicle. At this point, the rate of the wheel/axle combination, though still negative in value, is now moving positive in direction. The wheel/axle combination is thus accelerating so as to catch up to the speed of the vehicle. When the rate rises above a second predetermined (negative) threshold, the wheel slip controller commands the slip control valve to the lap position thereby maintaining whatever pressure is currently in the brake cylinders of the truck. Once the wheel/axle combination reaches the speed of the vehicle, its rate of change, though positive in value, is now again moving negative in direction because the vehicle has a negative acceleration due to the braking. When the speed of the axle matches that of the vehicle and its rate of change indeed moves in the negative direction, the wheel slip controller commands the slip control valve to the apply position thereby reapplying the brakes to the wheels of truck. Should wheel slip be detected again, the system will again correct it as noted above. It is through such detection and correction that the patented system continually addresses the problem of wheel slippage.
As is known in the transit industry, a passenger transit rail vehicle can be decelerated using either dynamic braking or friction braking or a combination of both. Such a transit vehicle typically has two axles per truck. Each axle on each truck typically has its own AC motor through which it may be propelled and dynamically braked. A single propulsion brake controller typically has (per truck) control over the two AC motors on its truck thereby electrically coupling the axles on the truck and enabling them to operate essentially in unison during propulsion and dynamic braking. The two axles of the truck likewise have friction brake control equipment in common according to typical practice in the transit industry.
There are two modes of operation for the brakes on a truck in such a passenger transit vehicle. The brakes operate in the decoupled mode when the wheel/axle combinations on the truck are braked using only the friction brake control equipment. During the decoupled mode of operation, the axles on the truck respond to such braking essentially independently. The brakes operate in the coupled mode of operation when the AC motors are used to electrically brake the axles on the truck. During the coupled mode, the propulsion brake controller simultaneously controls both AC motors and thus both axles on the truck. Though electrically coupled, the axles of the truck act as if they are mechanically coupled and operate essentially in unison during dynamic braking.
Many wheel slip control systems work in conjunction mostly with the friction brake control equipment, and merely remove dynamic braking until the wheel slipping problem has been corrected. For example, the slip control valve taught in U.S. Pat. No. 4,491,920, supra, can be commanded to the release, lap and reapply positions accordingly so as to correct the wheel slip that occurs during friction braking. Dynamic braking on the truck, though, may be addressed only to the extent that the propulsion brake controller controlling the AC motors is prevented from electrically braking the axles on the truck while the wheel slip is being corrected. Rail vehicles configured in this manner essentially have their wheel slip control systems alternately apply and release rapidly the friction brakes so that the affected wheel/axle combination experiences an averaged, reduced braking force.
Many other wheel slip control systems correct wheel slip by affecting the operation of both the friction braking and the dynamic braking equipment on the truck. An example of such a wheel slip control system appears in U.S. Pat. No. 5,752,212, entitled PROPORTIONAL POLARITY WHEEL SLIDE PROTECTION. This application is assigned to the assignee of the present invention, and incorporated by reference into this document. This particular system allows the dynamic and friction brake systems to operate basically in harmony while wheel slip is being controlled. It is incorporated into the overall brake control system so that the same devices used to modulate the braking force for normal vehicle braking will also be used to modulate the braking force during wheel slip control. Generally stated, this system directs both the dynamic brake equipment and the friction brake equipment to reduce, proportionately, the braking force that it applies to the axles of the affected truck. It does this by taking into account data such as the speed and the acceleration/deceleration rate of the wheel/axle combinations, the severity of the wheel slip and the duration of the wheel slip. This system controls wheel slip whether the vehicle is being decelerated using either dynamic braking or friction braking or a combination of both.
The wheel slip control system described in U.S. Pat. No. 4,941,099, entitled ELECTRONIC ADHESION ADAPTIVE WHEEL SLIDE PROTECTION ARRANGEMENT FUNCTION., is another example of a wheel slip control system that affects the operation of both the friction braking and the dynamic braking equipment on a truck. This patent is assigned to the assignee of the present invention, and incorporated by reference into this document. This particular system uses two detection circuits, a primary wheel slip detection circuit and a synchronous wheel slip detection circuit, each employing its own separate pattern recognition slip control logic. This system combines the separate slip control logic using a relatively complex scheme to correct slipping of the wheel/axle combination by modulating the force applied by the brakes to the affected wheel/axle combinations on the truck.
More relevant to the present invention is that the system described in U.S. Pat. No. 4,941,099 employs detection and correction parameters through which it detects and subsequently corrects slipping of the wheel/axle combinations on the truck. Like in other wheel slip control systems in the art, the parameters used by this patented system are stored in memory devices either individually or in the form of tables or both. The detection and correction parameters are constants each of which is generally preselected to work in connection with the circumstances of the particular application for which it was intended.
There are many wheel slip control systems that employ such detection and correction parameters according to principles known in the wheel slip control art. The detection parameters are used by the detection logic embodied in the programming code, whereas the correction parameters are used in the correction logic in the code. For example, as a microprocessor executes the instructions embodied in the programming code, the wheel slip controller monitors various input signals such as rotational speeds and the rates at which the wheel/axle combinations of the truck accelerate or decelerate. From these input signals and others, it may derive signals indicative of the vehicle speed, vehicle deceleration and other required information the nature of which depends on the particular wheel slip control system at issue. Based on such signals, the wheel slip controller retrieves the specific detection parameter(s) appropriate to the current operating conditions. Simply stated, the wheel slip controller uses the detection parameters in its detection logic when determining whether any of the wheel/axle combinations on the truck are actually experiencing slip. Similarly, once wheel slip is detected, the wheel slip controller retrieves the specific correction parameter(s) appropriate to the degree of slip experienced by the affected wheel/axle combination. It is these selected correction parameter(s) that the wheel slip controller uses in its correction logic when correcting the slip, i.e., temporarily reducing the braking force applied to the slipping wheel until such slipping ceases or reduces to an acceptable level.
The primary disadvantage common to all known prior art wheel slip control systems is that the detection and correction parameters they use are optimized only for the coupled mode of operation. Although this prior art approach enables a wheel slip control system to avoid lockup of the wheels in the decoupled mode of operation, overall stop performance suffers. A system employing this approach does not decelerate and stop a rail vehicle as efficiently as would be possible in a wheel slip control system featuring the present invention.
A related disadvantage common to all known prior art wheel Slip control Systems is that they do not implement the programming logic necessary to select the appropriate detection and correction parameters based on whether the axles of a truck are operating in the coupled or decoupled modes. Such programming logic, of course, is presented as part of the present invention.
There are other wheel slip control systems and methods that, in whole or in part, relate to the present invention. One such system is described in U.S. Pat. No. 5,471,387, entitled METHOD OF AND APPARATUS FOR THE COMBINED DETECTION OF SPEED VARYING ENERGY LEVEL WHEEL SLIP DETECTION AND DETERMINATION OF WHEEL SLIP INTENSITY OF A RAILWAY VEHICLE BRAKE SYSTEM. Another such system is described in U.S. Pat. No. 5,654,889, entitled SIMPLIFIED PATTERN RECOGNITION WHEEL SLIDE PROTECTION. The inventions described in these two documents are assigned to the assignee of the present invention, and both are incorporated by reference into this document.
It should be noted that the foregoing background information is provided to assist the reader in understanding the present invention. Accordingly, any terms used herein are not intended to be limited to any particular narrow interpretation unless specifically stated otherwise in this document.